[vectorit]

Welcome!
In a short answer to your question: Yes, you can convert from manual to power steering. But, it's not a simple bolt on process of swapping boxes.

The frame on your 68 will more than likely have a flat mounting surface on the frame, and the power boxes need contours in the frame similar to dimples.

There are a few options in getting this swap to work, such as adapters and beating the frame to make it work...

I'm still running my manual gear box since I'm not comfortable with a lot of the options to make the conversion possible, but that's mainly because I am more concerned about originality.

Others should chime in sooner or later.